The rapid growth of international trade in chemicals in recent years has let to the development of specialized cargo vessels--chemical tankers--for the expeditious and economical transport of liquid chemicals by water. Such vessels carry their cargo in tanks forming integral parts of the ship corresponding to the holds of conventional vessels. Typically, a chemical tanker will have a plurality of tanks, the contents of which may differ. A given tank may contain a different chemical on each voyage.
Typically, the chemical tanks open to a deck for loading and unloading through a hatch. The cover of this hatch must secure the tank--both against spillage of the contents and contamination by water and other materials from without. Essential to the sealing effectiveness of the hatch cover is a gasket, normally held in a groove in apposition to the lip or outer circumference of the hatch cover. This gasket must keep the hatch cover "tight." Similar conditions occur on tank trucks and railway tank cars, and the present invention is suitable to such applications as well.
The desired characteristics of such a gasket are: a. Resistance to the widest variety of chemicals, since the gasket must encounter in random sequence a large number of possible chemicals. b. Durability. The gasket must be sturdy enough to maintain an unbroken seal while withstanding repeated closings of the heavy hatch cover. c. Resilience. The gasket must be sufficiently flexible to seat well in a channel. To retain its sealing effectiveness, the gasket should also be able to recover from impact so that by regaining its shape, it will not be permanently compressed or "set" and thus will seal through repeated closings.
Hatch cover gaskets currently available are lacking in one or more of the desired characteristics described above. Polytetrafluoroethylene (TFE) fiber packings, for instance, will withstand virtually all chemical attack, but they are not sufficiently resilient for this application, and they are also subject to cold flow, which would tend to extrude them from the channel in which they are seated. TFE-impregnated asbestos packings have lower chemical resistance than TFE fiber and are less durable. Also, they have no resilience. Additionally, in view of the well-known carcinogenic properties of asbestos, many operators would prefer to have an asbestos-free product which could not conceivably contaminate the contents of the tank or create any other undesirable environmental effect. Rubber packings are resilient and durable, but they will not withstand many of the solvents used to wash out tanks, nor will they withstand certain frequently transported chemicals.
It would be costly and impractical for the ship owner to inventory a multiplicity of gasketings for use on specific cargos, and costly and impractical for the crew to have to change each gasket on a number of tanks to other types of gaskets every time the tank is cleaned or a new cargo is loaded.
Accordingly, there is a need for an improved packing for chemical tank lids on ships, trucks and trains, wherein said packing embraces all of the desirable characteristics described above, and it is the purpose of the present invention to provide such an imperatively needed product.
This invention discloses a gasket or packing material which will resist virtually every chemical carried in the typical steel chemical tank, which is strong enough to withstand repeated closures of the hatch or tank cover, and which has the ability to recover from the deformation of the hatch cover after the lid is raised.